This invention relates to a method and apparatus for determining the temperature of a sample by measuring the specular reflectance from the sample.
The concept of optical bandgap thermometry is at least twenty five years old. In 1972 H. Wieder in "Novel method for measuring transient surface temperatures with high spatial and temporal resolution," J. Appl. Phys. 43, 3213 (1972) proposed that the temperature of a semiconductor sample could be deduced from the temperature dependence of its own optical bandgap. He developed a technique that he called the laser thermoprobe as he published in "Laser thermoprobe," Opt. Comm. 11, 301 (1974) and in "Thermal-profile measurements with submicrometer resolution," Laser Focus 11, 86 (May, 1975). As the sample temperature changed, he measured the change in the reflected intensity of a laser beam whose energy lay within the range of the absorption edge. Subsequently, it appears that four other groups may have independently suggested the same idea. See D. A. Christensen, "A new non-perturbing temperature probe using semiconductor band edge shift," J. Bioengineering 1, 541 (1977); K. Kyuma, S. Tai, T. Sawada and M. Numoshita, "Fiber-optic instrument for temperature measurement," IEEE J. Quant. Elec. QE-18, 676 (1982); E. S. Hellman and J. S. Harris, Jr., "Infra-red transmission spectroscopy of GaAs during molecular beam epitaxy," J. Crystal Growth 81, 38 (1987); and J. C. Sturm, P. V. Schwartz, and P. M. Garone, "Silicon temperature measurement by infrared transmission for rapid thermal processing applications," Appl. Phys.
Currently there are two major applications of optical bandgap thermometry. One is based on either normal incidence transmission or reflection. The other depends on diffuse reflection from the back side of a sample.
The transmission method was developed to measure wafer temperature in molecular-beam-evaporation thin-film growth systems. See D. Kirillov and R. A. Powell, MIMIC Phase III Contract No. DAALOI-89-C-0907, final report, April 1991 and their U.S. Pat. No. 5,118,200, and M. E. Adel, Y. Ish-Shalom, S. Mangan, D. Cabib, and H. Gilboa, "Noncontact temperature monitoring of semiconductors by optical absorption edge sensing," SPIE 1803, 290 (1992) relating to the transmission method. See also U.S. Pat. No.4,136,566 to Christensen which appears to correspond to the 1977 Christensen paper and where the detector detects intensity of transmitted radiant energy. The essential elements of the transmission method are depicted in FIG. 1(a). The heater serves a dual purpose: 1) it directly heats the wafer radiatively to the growth temperature and 2) it serves as a built-in light source for the transmission measurement. The light that passes through the wafer is directed to a spectrometer. The spectrometer is scanned over a photon energy substantially range that straddles the bandgap energy.
An illustrative transmission spectrum, which has a sigmoidal line shape, is shown in FIG. 2. For photon energies below the bandgap energy, E.sub.g, the transmittance is of the order of 50%. Just below E.sub.g, the transmittance abruptly drops to zero. This steep edge is commonly referred to as the bandgap absorption edge.
The bandgap energy depends on sample temperature, decreasing as the sample temperature increases. See C. D. Thrumond, "The standard thermodynamic functions for the formation of electrons and holes in Ge, Si, GaAs, and GaP," J. Electrochem. Soc. 122, 1133 (1975). Thus, the position of the absorption edge shifts to lower energy as the temperature increases as seen in FIG. 8 to be discussed below. The sample temperature can then be inferred from the position of the absorption edge.
The transmission method works well for situations in which the sample temperature is high (&gt;500.degree. C.). The heater is a bright light source because its temperature is high. Film growth on wafers whose temperature is lower than 500.degree. C. requires cooler heaters so that the brightness is too low. One way to remedy this situation is to place an ancillary light source behind the sample. However, in many growth systems this may be technically impractical.
The salient features of the diffuse reflection method have been described by M. K. Weilmeier, K. M. Colbow, T. Tiedje, T. Van Buuren, and L. Xu, "A new optical temperature measurement technique for semiconductor substrates in molecular bean epitaxy," Can. J. Phys. 69, 422 (1991) and by S. R. Johnson, et al., in their U.S. Pat. Nos. 5,568,978 and 5,388,909 and these features are pictured in FIG. 1(b). To enhance the coupling between the heater radiation and the sample, the back surface of the sample is often roughened. Light from a broadband source is directed to the front of the sample. Part of the light crosses the front surface and propagates to the back surface. Part of this light diffusely reflects (i.e., scatters in all directions back into the sample) from the rough surface. The diffusely reflected light is collected in a non-specular direction with a lens and focused into a spectrometer. Diffuse reflection spectra are recorded as in the transmission method and a line shape similar to that in FIG. 2 is obtained.
The diffuse reflection method is another solution to the low light level problem. However, the signal-to-noise ratio is much worse because the intensity of the detected light below E.sub.g is very low. In addition, it is difficult to accurately model the diffuse reflection. The angular dependence and magnitude of the scattering are not known and it is difficult to account for the collection of the rays.
In many situations it is not feasible or desirable to roughen the back surface of the sample. In process chambers, such as plasma etching systems, samples may be attached to holders that can be either heated or cooled. Transmission is not possible and diffuse reflection is not practical because back side is specular.
In a hybrid mode, the Christensen U.S. Pat. No. 4,790,669 relates to optical bandgap thermometry which records a spectrum either by normal-incidence reflection from the sensor or by normal-incidence transmission through the sensor. FIG. 9(b) uses normal-incidence reflection from the thick film backed by a mirror. FIG. 10(b) is similar to FIG. 9(b) except that the effect of the mirror has been replaced by total reflection at the beveled sides of the detector. FIGS. 11 (b) and 12(b) are also similar to FIG. 9(b) except in the manner in which the light is reflected from the back side of the sensor; this may affect the value of the reflectance at photon energies below the bandgap energy. FIG. 13 uses normal-incidence reflectance from a semi-infinite sample, which will not work as a temperature sensor. FIG. 10(a) is basically a transmission configuration and FIG. 14 uses normal-incidence transmittance. There is no focus on using the reflectance of specular light at nonnormal incidence.